Hi Gerd, Thanks for your response.
I created a PR as you suggested: https://github.com/tianocore/edk2/pull/12997 There is couple of check failed, I hope it is transient due to infrastructure. Please re-run the checks. Thanks Ramkumar -----Original Message----- From: [email protected] <[email protected]> Sent: Thursday, August 20, 2026 7:28 PM To: Ramkumar Santhanakrishnan <[email protected]> Cc: [email protected]; [email protected]; [email protected]; Ashish Gupta (SJC) <[email protected]> Subject: Re: Regarding the dynamic MMIO window size !-------------------------------------------------------------------| CAUTION: External Email |-------------------------------------------------------------------! On Thu, Aug 20, 2026 at 08:25:30AM +0000, Ramkumar Santhanakrishnan wrote: > Hi > > I have a question about commit > ecb778d0ac62560aa172786ba19521f27bc3f650<https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_tianocore_edk2_commit_ecb778d0ac62560aa172786ba19521f27bc3f650&d=DwIBAg&c=s883GpUCOChKOHiocYtGcg&r=bgO5TLA4ABSXtFOlOt6oU_TMINnYWY2E7WIcTKTpOtQ&m=7YjJxmPE8qmKci4uyUt7t0IUdW1Q9ibrdhuRr881Lk9sObxHxPzXyRy5CfZoEB5R&s=XB1Ad7-LMa_nx3HdBvb8SIDGTjQCcdaJ1kdVJ58Jh4E&e= > >, which limits the MMIO window to 1/8 of the total physical address space. > For a guest with 46-bit physical addressing, this gives an 8 TB MMIO > window. We cannot increase the guest physical address width or move to > 5-level paging due to compatibility constraints. With newer GPUs (up > to 512 GB VRAM each), 8 TB may be insufficient for passthrough of 8 > GPUs, mainly because each BAR requires base address alignment. So you need 1 TB address space per GPU because of alignment I assume? So 8 TB total, and then a bit more for the non-GPU devices (network, storage etc)? > We are evaluating increasing the MMIO window size. > Could you share the rationale for the 1/8 limit? I've made it depend on the total address space available so it scales up automatically (newer + larger machines get larger MMIO windows), hoping that this logic works unchanged for a while. Using 1/8 looked like a generous number when introduced, which was before GPUs and NPUs with huge device memory appeared on the market. > Do you see any concerns with changing it to 1/4, or making it > configurable from QEMU? It is configurable via fw_cfg as already mentioned in the thread. If this is not working for large window sizes this should be fixed. I don't expect problems when going from 1/8 to 1/4 as default size, so no concerns here. Feel free to open a PR (best with your test results in the description). take care, Gerd -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#122116): https://edk2.groups.io/g/devel/message/122116 Mute This Topic: https://groups.io/mt/120843033/21656 Group Owner: [email protected] Unsubscribe: https://edk2.groups.io/g/devel/unsub [[email protected]] -=-=-=-=-=-=-=-=-=-=-=-
